White Paper
Quality of Service on Cisco Catalyst 6500
INTRODUCTION
Picture a busy bank branch. The queue of customers lining up to see the tellers is very long. When you arrive at the bank branch, you are
immediately recognized as one of the bank's valued customers. You are escorted to a separate queue for "special" customers away from the hustle
and bustle of the long queue. In this special queue you get served by the teller immediately. As a special customer, you have just been given
a preferential level of service that exceeds what the average customer receives.
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Like in the bank example above, quality of service (QoS) in Cisco
Catalyst
6500 Family line cards is a tool that is used to provide preferential
service for select traffic as it transits the switch. Over time and with the advancements in hardware and software technology, a number of QoS tools
have now become available. QoS in itself is not one feature, but a collection of features that when combined provide a powerful way to identify
different classes of traffic, prioritize them, and then service that traffic ahead of other lower priority traffic entering and leaving the switch.
This document will attempt to provide a high level overview of the QoS features found on the Cisco Catalyst 6500. It will explain what the features
are, how they work, and where in hardware they are performed.
WHERE IS QoS PERFORMED?
The Cisco Catalyst 6500 performs QoS on the supervisor and the line card. The supervisor contains a daughter card called the policy feature card
(PFC). Although the PFC is primarily responsible for the hardware forwarding of packets, it also performs a number of important QoS tasks. Since
the Cisco Catalyst 6500 was introduced in 1999, a number of PFC models aligned with specific supervisor models have been introduced. Typically
the introduction of a new PFC coincides with the arrival of new QoS features. This is especially true of the PFC3, which when introduced with the
Cisco Catalyst 6500 Series Supervisor Engine 720 in 2003 added a number of new QoS features not found in earlier PFC models.
The Cisco Catalyst 6500 line card is the other component that performs QoS, and those QoS features are primarily influenced by the port
application-specific integrated circuit (ASIC). The level of QoS support on the line card is dependent on the functionality built into the line-card port
ASIC. For this reason, the QoS capabilities can differ between different generations of the Cisco Catalyst 6500 Family.
Toward the end of this paper, a set of tables give an overview of the QoS features available on each of the line cards and PFC versions.
A QUICK RECAP ON SETTING PRIORITY
When data is sent through a network, it can be tagged with a priority value. When the data passes through a network device, the priority value is
used by that network device to determine how it should treat the packet. Data can be tagged with a priority value as described in the following
sections.
Class of Service
When a packet is transmitted out an Ethernet port, it has an Ethernet header attached to it. This Ethernet header can include an optional VLAN tag
(also referred to as an IEEE 802.1Q VLAN tag). Within the VLAN tag is a 3-bit field called the class-of-service (CoS) field. These 3 bits can be
manipulated to yield eight different priority values. Figure 1 shows where in the Ethernet header the priority bits are found.
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